A chuck for use in the headstock of a lathe normally has a chuck body which is centered on and rotatable about an axis, and a plurality of jaws which are radially displaceable on this chuck body to hold a workpiece being machined in the lathe. Although chucks exist wherein the jaws are independently radially displaceable, normally these jaws are three in number and are jointly radially displaceable so that a workpiece gripped thereby will be centered on the chuck axis.
In one known system as seen for example in German patent publication No. 2,232,022 axially backwardly projecting formations on the backs of the jaws engage in spiral formations on a closing plate carried in the chuck body and rotatable about the axis of the chuck to displace the jaws radially jointly inwardly and outwardly. This ring is normally manually rotated to grip and release the workpiece.
It is also known from German patent publication Nos. 1,940,609, 2,007,509, 2,347,561, and 2,533,803 to radially displace the jaws by means of rods axially or tangentially engaged in the chuck body and in turn engaging the backs of the jaws by means of teeth or wedge formations so that axial or tangential shifting of these rods radially displaces the chuck jaws. Such rods are in turn operated manually or by means of external fluid-operated cylinders or lever assemblies.
In another known arrangement seen in German patent publication No. 2,060,057 a lever pivotal in the chuck body is employed to adjust the jaws, and German patent publication No. 2,052,037 teaches the use of axially displaceable so-called wedge hooks which engage the jaws.
In all such arrangements changing the jaws is a relatively difficult task. Such changing is necessary to use jaws particularly set up to accommodate a particular kind of workpiece, or to replace jaws which are badly worn or which have been damaged during the machining operation. To this end the jaws are typically made of mild steel, whereas the rest of the chuck is made of very hard tool steel.
In the system of German patent publication No. 2,232,022 it is necessary to rotate the so-called spiral ring through many revolutions in order to move the jaws radially outwardly far enough so that they can be removed from the chuck. Such an operation is relatively time consuming, and fitting of the new jaws back into the chuck is a relatively difficult operation as it is essential to ensure that the jaws are properly radially positioned for good centering of a workpiece to be gripped thereby.
The system of German patent publication Nos. 2,347,561 and 2,533,803 requires a relatively long travel of the operating mechanism in order to free the jaws so that they can be removed from the chuck. Thus it is necessary to dimension the assembly so that the actuating member can move far enough back to completely disengage the operating rods from the jaws for their removal. At the same time some mechanism must be provided to prevent accidental displacement of the operating member into this fully withdrawn position, as should it accidentally move into this position during operation of the lathe the jaws will fly outwardly from the chuck and create a considerable hazard.
It has been suggested in the wedge-hook system of German patent publication No. 2,052,037 to make the wedging arrangement limitedly angularly dispalceable so that in one of its angular positions it is completely out of line with the jaws and their removal is relatively easy. Such an arrangement has the advantage of simplicity, that nonetheless requires replacement of the entire relatively expensive jaws. Furthermore such an arrangement has the disadvantage that maladjustment can result in the jaws not being adequately held in the chuck so that they can fly therefrom on rotation of the chuck as described above.
Thus in virtually all of the known systems replacement of the chuck jaws is a relatively difficult and expensive operation. For this reason when changing over from one type of machining operation to another, when the changeover requires replacement of the jaws, considerable downtime must be reckoned with for rebuilding of the lathe chuck.